docs: def fromSuccessResponse(cls, success_response, signed=True):
          """Construct a FetchResponse object from an OpenID library
          SuccessResponse object.
  
          @param success_response: A successful id_res response object
          @type success_response: openid.consumer.consumer.SuccessResponse
  
          @param signed: Whether non-signed args should be
              processsed. If True (the default), only signed arguments
              will be processsed.
          @type signed: bool
  
          @returns: A FetchResponse containing the data from the OpenID
              message, or None if the SuccessResponse did not contain AX
              extension data.
  
          @raises AXError: when the AX data cannot be parsed.
          """
          self = cls()
          ax_args = success_response.extensionResponse(self.ns_uri, signed)
  
          try:
              self.parseExtensionArgs(ax_args)
          except NotAXMessage, err:
              return None
          else:
              return self